For the people out there who still have Windows XP deployed, it is worth noting that you can’t install the Enterprise Vault Outlook Addin on XP SP 2 or lower. The minimum required version is Windows XP SP 3. It is shown in the Release Notes :

Service Pack 3 is the new minimum version of Windows XP [Ref 90310584]

Windows XP SP3 is the new minimum version that is supported for use with the following: